Scientists study centuries-long Sierra dry spell

By Mark Grossi / The Fresno Bee
Posted at 10:07 PM on Sunday, Mar. 28, 2010 Scientists have found evidence in Sequoia National Park of a centuries-long dry spell — and clues about how the Sierra Nevada could be changing. The researchers studied tree rings on dead giant sequoias, the largest trees on Earth. They found that during a warm, dry period between A.D. 800 and 1300, fires were more frequent, suggesting more fires may be ahead for a Sierra facing similar conditions today. Their findings have been published in the most recent edition of the journal Fire Ecology, said Thomas W. Swetnam, lead researcher and director of the Tree Ring Laboratory at the University of Arizona in Tucson. Information about how sequoias responded to the 500-year warm spell is important because scientists predict climate change may subject the forest to a similar environment again, Swetnam said. One lesson: the Sierra will see more fires. The study also answers doubts about whether there really was a 500-year warm period in the Western United States, he said. “It is thrilling to see 3,000 years of history recovered from these amazing trees,” Swetnam said. “This is the longest tree-ring history that’s been established in science.” Swetnam and his collaborators, including fire ecologist Anthony Caprio in Sequoia National Park, focused on samples from 52 dead and downed sequoias in Giant Forest. … Swetnam said the study shows fire was far more frequent in the sequoias during past warmups than it has been over the last several decades. If the warm, dry conditions continue, it could lead to more catastrophic fires in the dangerously overgrown Sierra forests. … “The past is not a perfect guide,” Swetnam said. “We are in a much different situation now compared to the past. Our point is that it’s likely we’re heading into something like that 500-year warm period.”